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How many visitors do you need for your A/B test?

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This presentation will help you understand which factors are important while calculate number of visitors needed for an A/B test.

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How many visitors do you need for your A/B test?

  1. 1. How many visitors do you need for your A/B test? testing simplified!© Wingify Software Pvt. Ltd.
  2. 2. Disclaimer • We can never say that we need X number of visitors for an A/B test – Theoretically, an A/B test can take infinite number of visitors before producing statistically significant result • Instead, what we can say is that we need to at least test X number of visitors – With Y probability (usually 80%) of detecting a statistical difference in results (if there is any)© Wingify Software Pvt. Ltd.
  3. 3. Phew! Explanations please..© Wingify Software Pvt. Ltd.
  4. 4. First, let’s define statistical significance • Conversion rate is never an exact number, it is always a range. That is, we can never say conversion rate is 10%, although we always say it is 10% ± 1% – This is because as we collect data, we are estimating what real conversion rate is (in statistical terms, we are estimating population mean from sample mean) – Initially, our guesses are raw (as we only have few data points) but as we test more visitors, the error range decreases and we have better estimates of conversion rate© Wingify Software Pvt. Ltd.
  5. 5. But, still, conversion rates are always estimates… • So, we now have two conversion rate estimates for control and variation, say following: • Observe how these ranges are overlapping, and so even if conversion of variation appears to be worse, we cannot say for sure (until these ranges are non overlapping, as following)© Wingify Software Pvt. Ltd.
  6. 6. So, how many visitors to A/B test? • There are two scenarios in A/B test: – Variation is performing better (or worse) as compared to control • Difference in conversion rate is statistically significant – Variation is performing similarly as compared to control • Difference in conversion rate is not statistically significant© Wingify Software Pvt. Ltd.
  7. 7. So, how many visitors to A/B test? • Aim of A/B test calculations is to make sure we test enough visitors in order to know with certain confidence whether there is any statistical difference in control and variation conversion rate • As stressed earlier, we can never be 100% sure that after testing X number of visitors we will know if test has a statistically better or worse performing variation – If we test even more visitors, there are even better chances of finding a statistical difference if it really is there© Wingify Software Pvt. Ltd.
  8. 8. Factors important in calculations • Suppose we want to calculate X, which is the number of visitors we need to test in order to find out whether statistical significance is there • There are various factors which help us calculate X: – Statistical Power (usually 80%): it is the probability with which you expect to find out statistical significance after testing X visitors. (There are 80% chances that after testing X visitors, we will find statistical significant results if they are there) • As statistical power increases, your chances of finding a statistical difference gets better (but of course you need to test more visitors)© Wingify Software Pvt. Ltd.
  9. 9. Factors important in calculations – Statistical confidence (normally 95%): once statistical difference is found, it is the confidence we have in that difference. (There is 5% chance that the difference in conversion rate is not real and is due to randomness) • If you need higher statistical confidence, we need to test more visitors – Existing conversion rate of website: for lower conversion rates websites (say ones with 1% conversion), we need to test many more visitors as compared to situation if average conversion rate is higher (say 10%) – Difference in conversion rate you want to detect: if you want to detect even a small difference in conversion rate (say you want to know if variation differs from control by even 0.1%), you need to test many more visitors. If you are only concerned with detecting a large differences (say only >10%), you need to test lesser number of visitors – Number of variations you are testing: obviously, if you are testing 4 variations you need twice the number of visitors as compared to situation when you are just testing 2 variations.© Wingify Software Pvt. Ltd.
  10. 10. You need a thumb rule? Sorry, there is no thumb rule to find out how many visitors you need to test :(© Wingify Software Pvt. Ltd.
  11. 11. Not all is lost, though… • You don’t have to be a statistician in order to do these calculations, you can use an online calculator to find out number of visitors to test: http://visualwebsiteoptimizer.com/ab-split-test-duration/© Wingify Software Pvt. Ltd.
  12. 12. Questions? Paras Chopra, CEO, Wingify paras@wingify.com© Wingify Software Pvt. Ltd.

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